In the field of electrochemical devices including a an electrolyte, it is strongly desired that the electrolyte be solidified. Conventionally, an electrolytic solution is employed in batteries serving as electrochemical devices. Therefore, not only the leakage of the electrolytic solution, and the drying of the electrolytic solution within the battery due to the evaporation of the electrolytic solution occur, but also the partial drying of the separator due to the imbalanced presence of the electrolytic solution in the battery cell causes an increase in the internal impedance or an internal short circuit in the battery. Furthermore, in the field of solid electrolytes for use in a display element for electrochromic devices, a solid electrolyte which sufficiently satisfies the requirement for the operational speed has never been obtained. As countermeasures for solving these problems, the use of polymeric solid electrolytes has been proposed. Specific examples of such polymer electrolytes are solid solutions of matrix polymers containing oxyethylene chains or oxypropylene chains, and inorganic salts. These are complete solids and have excellent machining properties. However, the ionic conductivities thereof are 10.sup.-5 S/cm at room temperature, which are about 3 orders less than those of ordinary non-aqueous electrolytic solutions. As a method of improving the low ionic conductivity, the use of a polymer solid electrolyte film with a thickness in the order of microns has been proposed. However, it is difficult to control the thickness of the micron-order thick polymer solid electrolyte film in such a manner that the electric field in the battery cell is kept constant. The reliability of the battery cell obtained is therefore low.
In order to improved the ionic conductivity of a polymer solid electrolyte, a method of making a polymer solid electrolyte semi-solid by dissolving the same in an organic electrolytic solution (Japanese Laid-Open Patent Application 54-104541) and a method of polymerizing a liquid monomer with the addition of an electrolyte to produce a cross-linked polymer including the electrolyte (Japanese Laid-Open Patent Application 63-94501) have been proposed. However, the former method has the problem that the obtained solid electrolyte does not have sufficient solid strength, and the latter method has the problem that the ionic conductivity of the obtained cross-linked polymer including the electrolyte is not satisfactory.
The present invention solves the above-mentioned problems of the conventional polymer-based solid electrolytes, and provides a polymer-based solid electrolyte which not only has excellent ionic conductivity, but also excellent uniformity, and has a sufficient solid strength for use as a solid electrolyte for electrochemical devices, and a method of fabricating the solid electrolyte having such particular properties.